Add support for linear gradients, implemented as skia shaders.

sky/engine/core/painting/Point.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SKY_ENGINE_CORE_PAINTING_POINT_H_
 #define SKY_ENGINE_CORE_PAINTING_POINT_H_
 
 #include "dart/runtime/include/dart_api.h"
 #include "sky/engine/tonic/dart_converter.h"
 #include "third_party/skia/include/core/SkPoint.h"
 
 namespace blink {
 // Very simple wrapper for SkPoint to add a null state.
 class Point {
  public:
   SkPoint sk_point;
   bool is_null;
 };
 
 template <>
 struct DartConverter<Point> {
+  static Point FromDart(Dart_Handle handle);
   static Point FromArgumentsWithNullCheck(Dart_NativeArguments args,
                                           int index,
                                           Dart_Handle& exception);
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_POINT_H_